This invention relates to purification of mercaptoacyl amino acids. Exemplary of acids useful for purification by the method of the present invention are those disclosed by Ondetti et al., in U.S. Pat. Nos. 4,105,776 and 4,154,935 and in U.K. patent application No. GB 2028327 A. The mercaptoacyl amino acid of the Ondetti et al. disclosures are useful as hypotensive agents through their inhibition of angiotensin converting enzyme.